Solid electrolyte ionic conductor materials that transport oxygen ions appear to be very useful for the separation of oxygen from gas mixtures, for example, air. Certain of these oxygen ion transport materials are mixed conductors, that is, they conduct both oxygen ions and electrons. At elevated temperatures (generally greater than 450.degree. C.), oxygen ion transport materials contain mobile oxygen ion vacancies that provide conduction sites for selective transport of oxygen ions through the material. The ion transport is driven by the ratio of partial pressures of oxygen across the membrane: oxygen ions flow from the side with high oxygen partial pressure to the side that has a low oxygen partial pressure. Ionization of oxygen to oxygen ions takes place on the "cathode-side" of the membrane and these oxygen ions are transported across the oxygen ion transport membrane. The oxygen ions deionize on the "anode-side" and are released as oxygen molecules. For materials that exhibit only ionic conductivity, external electrodes are placed on the surfaces of the electrolyte and the electronic current is carried in an external circuit in an electrically-driven mode. In contrast, electrons are transported to the cathode internally in mixed conducting materials in a pressure-driven mode, thus completing the circuit and obviating the need for external electrodes. Mixed conductors, however, can also be used in electrically-driven mode, although it is desirable to do so only when the electronic conductivity is limiting.
Owing to their infinite selectivity for oxygen transport, oxygen ion transport materials have several potential uses in the area of air separation and purification of gases. Some applications of these oxygen ion transport membranes involve the use of an anode side reactive purge to improve ion transport-based processes for purification of oxygen-containing gases and for syngas, hydrogen and carbon monoxide production. The basic motivation behind using such a reactive purge is to reduce the oxygen partial pressure on the anode side of the oxygen ion transport membrane greatly by introducing an oxygen scavenging gas (for example, methane, methanol, ethanol, or hydrogen) for purification/separation operations. This reduction in the oxygen partial pressure enhances the pressure-driven oxygen transport through the oxygen ion transport membrane.
In processes where partial oxidation of fuels is desired, such as in syngas generation, employment of an oxygen ion transport membrane can take advantage of the low partial oxygen pressure generated on the anode by an oxygen consuming reaction, such as partial oxidation, to transport oxygen from a relatively low total pressure air stream to a high total pressure reaction site. This avoids a separate air separation plant and expensive compression system.
There are several potential problems with this basic approach. One problem, for example, is that reactively purged oxygen ion transport systems generally must deal with large amounts of heat generated in the oxygen ion transport module. Such heat release leads to undesirable exotherms in the oxygen ion transport module, and may damage its components.
A second difficulty is that all the fuel is introduced at one end of the oxygen ion transport module in a reactive purge process, while the oxygen is incrementally transported through the oxygen ion transport membrane along its entire length. As a consequence, the anode side gas composition is always fuel-rich near the fuel inlet and becomes increasingly fuel-lean as one approaches the other end of the oxygen ion transport module. This occurs irrespective of the overall fuel-to-oxygen ratio used in the oxygen ion transport module. Highly fuel rich operation at the purge inlet end leads to very low gas phase oxygen activity which could lead to corrosion or chemical decomposition of the membrane material. For example, in purification applications such as deoxygenation of oxygen-containing gases, this problem is most pronounced in the "inactive" region of the membrane at the purge inlet end, where no oxygen is transported through the membrane. Also, under some conditions (for example, high temperatures) fuel-rich operation with organic fuels could lead to coke or carbon formation which in turn could lead to fouling the oxygen ion transport membrane surface or the reactor and diminished performance of the oxygen ion transport module.
Similarly when the desired reaction on the anode is partial oxidation, such as in syngas production, unreacted hydrocarbon fuel species will be present on the anode leading to the possibility of solid carbon formation.
Another problem is that high overall fuel-to-oxygen ratios in the oxygen ion transport module will lead to incomplete combustion of the fuel, and cause the outgoing gas to contain species such as hydrogen, carbon monoxide and unreacted fuel which will adversely affect the fuel efficiency. In addition, a highly reactive gas such as hydrogen may be beneficial for effectively scavenging oxygen from the purge side of the oxygen transport membrane. Although hydrogen gas is generally more reactive than most organic fuels, its high cost and scarce availability make its use less desirable than carbon-containing fuels (for example, natural gas). Mazanec et al., U.S. Pat. No. 5,306,411, entitled Solid Multi-Component Membranes, Electrochemical Reactor Components, Electrochemical Reactors and Use of Membranes, Reactor Components, and Reactor for Oxidation Reactions, relates to a number of uses of a solid electrolyte membrane in an electrochemical reactor.
U. Balachandran et al., Dense Ceramic Membranes for Converting Methane to Syngas, submitted to the First International Conference on Ceramic Membranes, 188th meeting of the Electrochemical Society, Inc., Chicago, Ill. (Oct. 8-13, 1995), relates to the use of solid electrolyte transport membranes to convert methane to syngas.
E. A. Hazbun, U.S. Pat. No. 4,791,079, entitled Ceramic Membrane for Hydrocarbon Conversion, relates to the use of a solid electrolyte ion transport membrane for oxidizing hydrocarbons and dehydrogenation processes.
T. Nozaki and K. Fujimoto, Oxide Ion Transport for Selective Oxidative Coupling of Methane with New Membrane Reactor, AIChE J., Vol. 40, 870-877 (1994), relates to the oxidative coupling of methane in a solid electrolyte reactor to produce higher hydrocarbons.
H. Nagamoto et al., Methane Oxidation by Oxygen Transported through Solid Electrolyte, J. Catalysis, Vol. 126, 671-673 (1990), relates to the reactions of methane in a solid electrolyte ionic conductor and an analysis of the reaction products.
Prior art related to hydrocarbon conversion by partial oxidation in an ion transport module has been disclosed by ARCO, BP, and Argonne/Amoco (see citations above). In these prior art processes, air typically flows on the cathode side of the oxygen ion transport membrane, whereas a hydrocarbon gas stream is fed to the anode side of the membrane where the hydrocarbons react with oxygen permeating across the oxygen ion transport membrane. These processes, however, do not disclose the use of exhaust gas recirculation to obtain any benefits. In addition, these prior art processes are not intended for inert gas production or purification (for example, to produce nitrogen gas).
Purging of an ion transport membrane with sweep steam is disclosed in Kang et al., U.S. Pat. No. 5,562,754.
A tubular solid-state membrane module is disclosed in Dyer et al., U.S. Pat. No. 5,599,383, having a plurality of tubular membrane units, each unit having a channel-free porous support and a dense mixed conducting oxide layer supported thereon. The porous support of each unit is in flow communication with one or more manifolds or conduits to discharge oxygen which has permeated through the dense layer and the porous support.